Carbonate compensation depth

As shown in the diagram, biogenic calcium carbonate (CaCO3) tests are produced in the photic zone of the oceans (green circles).

In seawater, a dissolution boundary is formed as a result of temperature, pressure, and depth, and is known as the saturation horizon.

The sinking velocity of debris is rapid (broad pale arrows), so dissolution occurs primarily at the sediment surface.

Shells of dead calcareous plankton sinking to deeper waters are practically unaltered until reaching the lysocline, the point about 3.5 km deep past which the solubility increases dramatically with depth and pressure.

If the exposed sea bed is below the CCD tiny shells of CaCO3 will dissolve before reaching this level, preventing deposition of carbonate sediment.

The CCD is relatively shallow in high latitudes with the exception of the North Atlantic and regions of Southern Ocean where downwelling occurs.

This downwelling brings young, surface water with relatively low concentrations of carbon dioxide into the deep ocean, depressing the CCD.

In the Cretaceous through to the Eocene the CCD was much shallower globally than it is today; due to intense volcanic activity during this period atmospheric CO2 concentrations were much higher.

[7] Increasing atmospheric concentration of CO2 from combustion of fossil fuels are causing the CCD to rise, with zones of downwelling first being affected.